How to select or size a pump 101

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This blog breaks down the key information needed to properly select or size a pump for HVAC and hydronic systems. From flow rate and head to control strategy, materials, and code requirements, it outlines the critical considerations engineers and contractors evaluate during design and retrofit projects. Whether you’re specifying new equipment or replacing an existing pump, this guide helps ensure the right pump is chosen for the application.

Pumps

The minimum information needed to select or size a pump (plus the key terms).

1Required Flow Rate (GPM) 2Total Dynamic Head (TDH) 3Fluid Type 4Operating Temperature 5Motor Voltage / Phase / Frequency 6System Type 7Pump Configuration 8Redundancy Requirements 9Control Strategy 10NPSH Available (NPSHa) 11Materials & Construction 12Codes & Standards

Tip: On mobile, swipe left/right to see all topics.

Required Flow Rate (GPM): How much fluid needs to move

Typical sources

HVAC load calculations (tons to GPM)
Chilled water (roughly 2.4 GPM per ton)
Hot water (roughly 1–3 GPM per ton, depends on ?T)
Equipment submittals (AHUs, heat exchangers, boilers)
Process requirements (industrial systems)

Total Dynamic Head (TDH): How hard the pump has to push

This accounts for friction and pressure losses throughout the system.

TDH includes:

Pipe friction losses (supply + return)
Fittings (elbows, valves, strainers, coils, heat exchangers)
Equipment pressure drop
Static head (usually 0 in closed hydronic systems)

Measured in

Feet of head (most common)
or PSI (1 PSI ˜ 2.31 ft of head)

NOTE: TDH is one of the most commonly misunderstood inputs—if you’d like, we can help verify it before a selection is made.

Fluid Type: What you’re actually pumping

The fluid itself directly impacts pump sizing and performance.

Common examples:

Water
Water / glycol mixtures (20%, 30%, 40%)
Process fluids

Why it matters:

Density and viscosity directly affect pump performance
Glycol increases required head and reduces available flow
Seal and material compatibility must be verified

Operating Temperature

Temperature directly influences fluid behavior and pump component limits.

Especially important for:

Hot water systems
Boiler loops
Industrial and process systems

Why it matters:

Affects fluid properties
Impacts seal selection
Sets pump casing pressure ratings

Motor Voltage / Phase / Frequency

Electrical requirements can directly limit pump selection.

Common examples:

460V / 3-phase / 60 Hz
208V / 3-phase

This information often limits which pumps are available for a given application. While the items above represent the minimum information needed to select a pump, providing additional system details allows for a more refined selection—helping ensure the right pump is chosen, not just an acceptable one.

System Type

The type of system determines how head and air are handled.

Common system types:

Closed loop (most HVAC chilled and hot water systems)
Open loop (cooling towers, well water, process systems)

Why it matters:

Static head applies in open systems
Air handling and priming requirements change

Pump Configuration

How and where the pump is installed impacts performance and serviceability.

Common configurations:

In-line (vertical or horizontal)
End suction (base mounted)
Split-case
Vertical multi-stage

Configuration is usually driven by:

Required flow range
Required head range
Available space constraints
Maintenance preferences

Redundancy Requirements (Optional)

Redundancy improves reliability for mission-critical systems.

Common redundancy approaches:

Single pump
Duty / standby
Lead / lag
N+1

Common in:

Hospitals
Data centers
Central plants

This affects:

Pump type (twin pumps, parallel pumps)
Control strategy

Control Strategy

How the pump is controlled impacts sizing, efficiency, and integration.

How the pump will be controlled

Constant speed
VFD
ECM (smart pump)
BAS control (0–10V, BACnet, Modbus)

Why it matters:

Impeller sizing
Minimum speed limits
Energy efficiency
Integration with the BAS (your wheelhouse)

Additional Reliability & Mechanical Considerations

Factors that influence long-term performance, serviceability, and system uptime.

NPSH Available (NPSHa)

Ensuring adequate suction conditions protects pump reliability.

Critical for:

Large pumps
High-temperature applications
Open systems

Helps prevent:

Cavitation
Premature seal failure

Codes & Standards

Compliance requirements can dictate pump selection and materials.

Often driven by:

Local and state codes
Project specifications
Authority Having Jurisdiction (AHJ)

May require:

Specific materials or coatings
Efficiency or performance certifications
Documentation and traceability

Codes & Standards

Compliance requirements can directly impact pump selection.

May dictate:

Efficiency requirements (DOE, ASHRAE 90.1)
Pressure ratings
Seismic restraints

A final thought

Much of this information is determined by the engineer designing the system and is typically documented in the project’s specifications. When retrofitting an existing pump, many of these details can also be found directly on the pump’s data plate.

If you have any questions about pumps or product selection for any of the parts we sell, we’re here to help.